The luminal surface of the entire lower urinary tract including renal pelvis, ureter, bladder and proximal urethra is lined by a cell type described in standard textbooks as the transitional epithelium or urothelium, which is characterized by a highly specialized apical cell surface covered by rigid-looking urothelial plaques consisting of hexagonally packed 16 nm uroplakin particles. It has been suggested that urothelial defects can result in a loss of the permeability barrier function allowing the penetration of some urine irritants into the bladder wall thus causing pain in interstitial cystitis. In addition, it is generally assumed that urothelial cells lining the different portions of the urinary tract are the same; hence conclusions derived from studying cells of one region should be applicable to all urothelial cells. However, urothelia of renal pelvis/ureter/trigone are known to have a different embryological origin than urothelial cells of other sites, and there are indications that urothelium is biochemically heterogeneous. In this project, we will (i) determine whether the urothelia that cover renal pelvis, ureter, bladder, and proximal urethra actually consist of several distinct cell lineages by analyzing the relative contributions of intrinsic divergence vs. extrinsic modulation to urothelial phenotypes, and (ii) identify the stem cells in various urothelial compartments using cell kinetic, cell culture and stem cell transplantation techniques. These proposed studies can yield important information because if urothelium indeed can be separated into several distinct subpopulations belonging to different cell lineages, one needs to re-examine the validity of some earlier studies in which urothelial cells from different parts of the urinary tract were used interchangeably. Since stem cells are the preferred targets of carcinogens, gene therapy, and they are particularly suited for tissue engineering and tissue reconstitution, our studies can have practical implications on the source of urothelial cells for tissue engineering of various parts of the urinary tract, the cellular origin of in vivo urothelial wound healing following the surgical removal of the bladder, the possible involvement of urothelial cells in interstitial cystitis and in site-specific variations in bacterium: urothelium interactions, and the cellular origin of various urothelial tumors.